The present invention relates to a detector for scattered light as part of a hazard detector, particularly for detecting particles in a carrier medium, with a housing, with an inlet and an outlet in the housing, between which the carrier medium flows through the housing on a flow path, with a light source, which directs light to a scattered light centre, which lies on the flow path, with a receiver for a part of the light which is scattered onto particles in the scattered light centre, and with a light trap for light which is not scattered in the scattered light centre.
Such types of detectors for scattered light are known and serve, especially in aspiration fire alarm systems, to detect solid matter or liquid particles, in which the carrier medium consists of a representative partial quantity of the air of a room to be observed or of the device cooling air of a device to be observed. In an aspiration alarm system, this representative quantity of air is actively suctioned by means of a ventilator and fed into the inlet of the detector for scattered light. In devices to be monitored, such as for instance, EDP equipment or individual components thereof, as well as in similar electronic devices, such as for example, measuring, control and regulating devices, relaying equipment, and PBX devices, it is basically also possible to use the internal flow of the device-cooling air to feed a representative partial quantity of the device cooling air as carrier medium into the inlet of the detector for scattered light. An active suctioning ventilator is then unnecessary.
While the carrier medium flows through the scattered light centre on its flow path through the housing of the detector for scattered light, the light of the light source traverses the scattered light centre, and consequently, the carrier medium flowing through it, and, provided that it is not scattered onto particles in the carrier medium, is absorbed in the light trap opposite. The detector for scattered light is predominantly in this operating state. If the ray of light meets a particle, which could be, for example, a smoke particle or smoke aerosol, which provides the first indication of a fire in the initial stages, this particle diverts a fraction of the light as scattered light from its original direction, which is then absorbed by a highly light-sensitive receiver and whose intensity is measured by means of a subsequent evaluation circuit. If a certain threshold value of the light intensity is exceeded, an alarm is triggered.
Detectors for scattered light for detecting particles in a carrier medium are known from EP 0 756 703 B1 and EP 0 729 024 A2, in which the carrier medium flows through the housing in a longitudinal direction and either several light sources facing each other (EP ""703) or a receiver (EP""024) are arranged on the longitudinal wall of the housing. These known detectors for scattered light are disadvantageous in that, for one thing, in light sources opposite each other, there is a risk that a majority of the light of a light source sent is reflected on the glass body of an opposite light source and a part of this reflected light then falls unintentionally on the light-sensitive receiver, consequently making it more difficult to determine the scattered light portion. On the other hand, as far as the arrangement of the receiver on the longitudinal wall of the housing goes, it is disadvantageous that this is easily dirtied, since it is placed in the flow path, which could lead to reduced responsiveness or else to an increased error rate.
Detectors for scattered light of the type mentioned at the start are known from EP 0 463 795 B1 and WO 97/42485, in which the flow path of the carrier medium runs crosswise to the longitudinal direction of the housing, and consequently, crosswise to the receiver axis. The disadvantages of these known detectors for scattered light, in particular, are that the inlets and outlets placed crosswise to the housing with the feeding pipes for the carrier medium to be connected thereto do not facilitate either a compact construction of the detector for scattered light itself or its compact arrangement within a larger detector housing, in which, for example, an air current sensor and the evaluation circuit are also accommodated.
Finally, a scattered light measuring device of the type mentioned in the beginning is known from EP 0 257 248 A2, which exhibits a funnel or paraboloid-shaped light trap for light which is not scattered in the scattered light centre, with said light trap opening towards the light source.
The purpose of the present invention is to develop a detector for scattered light, of the type mentioned at the start, i.e., with a housing, with an inlet and an outlet in the housing, between which the carrier medium flows through the housing on a flow path, with a light source, which directs light on a scattered light centre, which lies on the flow path, with a receiver for a part of the light scattered in the scattered light centre onto particles, and with a light trap for light not scattered in the scattered light centre, in such a way as to ensure a compact structural shape and yet maintain high responsiveness.
This purpose is solved in a detector for scattered light of the previously described type with two alternative and highly advantageous embodiments of the light trap, as described in patent claims 1 and 2. According to a first alternative, it is provided for the light source to be placed outside the flow path, furthermore, for the centre axis of the light cone of the light source to run, at least partially, parallel in relation to or on the centre line of the flow path, and finally, for the light trap allocated to the light source to be part of the flow channel guiding the flow path. According to a second alternative, which can also be chosen cumulatively, the receiver is arranged outside the flow path, and the receiver axis runs, at least partially, in parallel in relation to or on the centre line of the flow path, and the light trap allocated to the receiver is part of the flow channel that guides the flow path.
The two embodiments according to the invention of the detector for scattered light lie are advantageous in that the light trap allocated to the light source, as well as the light trap allocated to the receiver, is at the same time a part of the flow channel that conducts the carrier medium, for example, the representative partial quantity of the device cooling air of an EDP device, on the flow path through the detector for scattered light. In the process, it is advantageous whenxe2x80x94as provided in an embodiment of the detector for scattered light according to the inventionxe2x80x94the flow channel exhibits a bend where it functions as a light trap, so that the flow path of the carrier medium is diverted, and consequently, the light source xe2x80x9clooks into empty spacexe2x80x9d towards the centre axis of its light cone and/or the receiver towards the receiver axis, as a result of which interfering reflections are excluded.
Advantageous embodiments of the invention are specified in the sub-claims.
First, two alternative embodiments of the shape of the light trap, which is allocated to the light source, are provided. According to a first alternative, this light trap is designed in such a way that, when seen from a cross sectional plane, which is vertically positioned on the receiver axis level formed by the receiver axis and the centre axis of the light cone of the light source, it exhibits the shape of a funnel, which opens towards the light source, and, cf. FIGS. 10 and 11, towards the receiver respectively. According to a second alternative, the light source is designed in such a way thatxe2x80x94again as seen in the previously described cross-sectional planexe2x80x94it approximately exhibits the shape of a parabola, whose opening points to the light source and, cf. FIGS. 10 and 11, towards the receiver. The advantages of the embodiment of the light trap according to the invention in both cases lie in the fact that light sent by the light source and not scattered in the scattered light centre is greatly reduced after repeated reflection on the walls of the light trap converging against the flow direction of the carrier medium, as a result of which it no longer affects the light-sensitive receiver, even at the highest sensitivity. As for the location of the receiver axis plane, it is to be assumed that this is horizontally aligned when the entire detector housing is on a horizontal plane. The cross-sectional shapes specified in the two alternative forms of embodiment of the light trap may involve the light trap being predominantly funnel-shaped or paraboloid-shaped, in which a sufficiently wide-open section for the entry of the carrier medium is of course provided towards the inlet.
For the shape of the light trap, it is furthermore provided for it to be designed crosswise to the described cross sectional plane, in such a way that it guides the flow path of the carrier medium in the receiver axis plane or parallel thereto in the bend through the scattered light centre to the outlet. Here, the arc-shaped curved guide on the inner wall of the light trap ensures that the deviation of the flow path from the inlet towards the scattered light centre is as free of turbulence as possible.
Since the design of the flow path for the carrier medium through the housing of the detector for scattered light has a great effect on the efficiency of the detector, the following four embodiments also deal with guiding the flow path. For one thing, it is provided for the centre axis of the light cone of the light source in the receiver axis plane to be directed towards an input channel or, alternatively, to an outlet channel, which connects in flow direction to the inlet and, with respect to the outlet channel, to the scattered light centre, and goes over to the light trap. For another, the shaping of the light trap for an increase in the sensitivity of the detector is of considerable significance. In this regard, a first embodiment of the detector for scattered light according to the invention provides for the light trap to run in an arc towards the centre line of the inlet channel and of the outlet channel respectively. Thus, the previously described attenuation of the non-scattered light portion, and with it, the detection certainty, is increased. Furthermore, it is advantageous when, in addition thereto, the flow path, after the inlet, initially runs parallel to the receiver axis before it leads through the scattered light centre towards the outlet after passing the inlet channel through the light trap in the arc. Finally, the flow path is diverted by at least 90xc2x0 before the inlet and/or after the outlet, but preferably twice. Each of these embodiments contributes towards avoiding the incidence of light not being scattered onto particles in the scattered light centre towards the receiver. A measurement for the sensitivity of a detector for scattered light is namely the so-called xe2x80x9cchamber valuexe2x80x9d, which is defined by the output signal of the light receiver in case there are no particles in the scattered light centre. The repeated changes in direction in the flow path are particularly advantageous, among other things, because, as a result, it prevents outside light from penetrating the scattered light centre when there is no suction pipe or no discharge pipe connected to the housing of the detector for scattered light.
The following embodiments deal with the light source, whose arrangement, formation, and orientation likewise have a great effect on the efficiency of a detector for scattered light. In order to reach a maximum responsiveness of the detector, a high light intensity is required, with said light intensity being reached in the present detector for scattered light preferably in that the light source exhibits two light emitters, which are arranged on top of each other in the previously described cross sectional plane, and are consequently arranged at the same angle to the receiver axis. Moreover, it is advantageous for the amount of light present in the scattered light centre when the two light emitters are arranged at a slope to the receiver axis plane, in such a way that their light cones cross in the scattered light centre. Again, each of the three embodiments contributes towards increasing the responsiveness of the detector according to the invention. Thus, this detector may, for example, also be used to monitor clean rooms (e.g., chip production), in which the smallest number of particles can tie up the production of chips for several weeks. In such areas of application, it is possible to increase responsiveness, providing that the technical possibilities of the detector allow it, because, in clean rooms, there is generally no occasion for deceptive alarms due to the lack of dust and lack of moisture. The arrangement of the two light emitters one on top of the other is not known in any of the described detectors for scattered light. When using several light emitters that can also be described separately from one another as individual light sources in a spatial separation, symmetrical arrangements around the receiver axis (EP ""703) or side-by-side arrangements are provided in the known detectors for scattered light. Both known arrangements of several light emitters have disadvantages. In the symmetrical arrangement around the receiver axis, there would have to be a screen for every light emitter, with said screen preventing direct light from shining on the receiver, in which the light of one emitter reflects on the screen of the other emitter, thereby unintentionally reaching the receiver, at least partially. The side-by-side arrangement of light emitters is disadvantageous in that the construction of the required screens and light traps would be more costly, and moreover, the light traps would have to be bigger in order to be able to catch both light cones.
Finally, with respect to the electromagnetic tolerance of the detector for scattered light, it may be advantageous when its housing is made of a synthetic material, which contains electrically conductive particles.
The previously described detector for scattered light may, for example, be part of a hazard detector in whose entire housing the housing of the detector for scattered light, also called xe2x80x9cdetector headxe2x80x9d, can be integrated. With respect to the housing of the detector, it is preferable for it to be made of three parts, namely one lower shell with an integrated flow channel for diverting the carrier medium into the flow direction behind the outlet of the detector head, furthermore, a cover for a part of the flow channel, and finally, an upper shell that functions as a housing cover for the danger warning system. Thus, the carrier medium flows only through the scattered light centre, the flow channel, and the suction source, which in the case of an aspiration fire alarm, could be a ventilator for suctioning the representative partial air quantity, for example. The electronics of the evaluation circuit, as well as the connecting terminals, remain outside of the sealed air conduction. A further advantage is the low manufacturing cost: the housing must be air-sealed only in the area of the air conduction, while a seal between the lower shell and upper shell is no longer necessary. The cable lead-ins also no longer need to be air-sealed. These advantages are particularly noticeable when using the detector for scattered light according to the invention in a harsh industrial environment with aggressive ambient air, if necessary. Examples for these are electroplating areas in circuit board manufacturing, lacquering lines, and battery production. In all these work areas, acids or flux accumulate in the ambient air, against which the sensitive evaluation circuit is to be protected. In this respect, it is advantageous when the flow path of the carrier medium is separated by a seal against the remaining components of the detector for scattered light, particularly against electronics and cables; a seal of the housing otherwise is not necessary. In the following, a first, a second, and a third embodiment of the detector for scattered light according to the invention will be explained more precisely using a drawing.
Shown are:
FIG. 1 a top view of the lower shell of the housing of the detector head of a first embodiment;
FIG. 2 a top view of the upper shell of the housing of the detector head of a first embodiment;
FIG. 3 a sectional top view of the upper shell with a view of the scattered light centre;
FIG. 4 a sectional view in the cross sectional plane 17 as per FIG. 3;
FIG. 5 a perspective representation of the housing of the detector head of the first embodiment;
FIG. 6 a view of the front side (top), a top view (centre), and a view of the rear (bottom) of a hazard detector housing;
FIG. 7 a top view of the lower shell of the hazard detector housing with flow channel, scattered light centre, and ventilator;
FIG. 8 a section along line Axe2x80x94A of FIG. 7;
FIG. 9 a section along line Bxe2x80x94B of FIG. 7;
FIG. 10 a top view of the lower shell of the housing of the detector head of a second embodiment; and
FIG. 11 A top view of the lower shell of the housing of the detector head of a third embodiment.